Highly efficient Ultra Super Critical Boilers, with advanced steam temperature and pressure, have recently been built in the world. Specifically, it has been planned to increase steam temperature, which was about 600° C., to 650° C. or more or further to 700° C. or more. Energy saving, efficient use of resources and the reduction in the CO2 emission for environmental protection are the objectives for solving energy problems, which are based on important industrial policies. And further, a highly efficient Ultra Super Critical Boiler and a furnace are advantageous for an electric power-generation and a furnace for the chemical industry, which burn fossil fuel.
High temperature and high pressure steam increases the temperature of a superheater tube for a boiler and a furnace tube for the chemical industry, and also a steel plate, a steel bar and a steel forging, which are used as heat resistant pressurized members, and the like, during the practical operation, to 700° C. or more. Therefore, not only the high temperature strength and the hot corrosion and steam oxidation resistance, but also the excellent stability of a microstructure for a long period of time, the excellent creep rupture ductility and the excellent creep fatigue strength are required for the steel used in such a severe environment.
An austenitic stainless steel is much better in the high temperature strength and the hot corrosion and steam oxidation resistance more than a ferritic steel. Accordingly, austenitic stainless steels can be used in high temperatures of 650° C. or more, where a ferritic steel cannot be used due to its strength and corrosion resistance. Typical austenitic stainless steels include 18 Cr-8 Ni type steels (hereinafter referred to as 18-8 type steels) such as TP 347H and TP 316H, and 25 Cr type steels such as TP 310 and the like. However, even the austenitic stainless steel has application limits in the high temperature strength and the hot corrosion and steam oxidation resistance. Further, although conventional 25 Cr type TP 310 steels have better hot corrosion and steam oxidation resistance than 18-8 type steels, they have lower high-temperature strength at temperatures of 650° C. or more.
Thus, various methods to improve both the high temperature strength and the hot corrosion and steam oxidation resistance have been tried. The following austenitic stainless steels have been proposed.
(1) Japanese Laid-Open Patent Publication No. 57-164971 discloses a steel in which the creep strength at a high temperature was improved by a combined addition of Al and Mg in addition to a large amount of N (Nitrogen).
(2) Japanese Laid-Open Patent Publication No. 11-61345 discloses a steel in which the high temperature strength and hot workability were improved by a combined addition of Al and N in addition to a suitable amount of B (boron), and by limiting the O (Oxygen) content to 0.004% or less.
(3) Japanese Laid-Open Patent Publication No. 11-293412 discloses a steel in which the hot workability was improved by a combined addition of Al, N, Mg and Ca, and by limiting the O (Oxygen) content to 0.007% or less.
(4) Japanese Laid-Open Patent Publication No. 2001-11583 discloses a steel in which precipitation strengthening or solid-solution strengthening was tried due to the nitride by addition of N, and the toughness of the steel used for a long period of time was improved by limiting the respective contents of Cr, Mn, Mo, W, V, Si, Ti, Nb, Ta, Ni and Co to specified levels or less, while associated therewith thereby to suppressing the precipitation of sigma phases without decreasing high temperature strength.
(5) Japanese Laid-Open Patent Publication No. 59-23855 discloses a steel in which the high temperature strength was improved by adding one or more of Ti, Nb, Zr and Ta in 1-13 times C content of in their total in a range of 1-10 times of C content, and making the microstructure of the steel a structure of No. 3-5 in the JIS austenitic grain size number.